In vertebrates, in addition to activating essential innate defense mechanisms, NF-kappaB/Rel factors are believed to function as central coordinating regulators of adaptive immunity. However, the direct assessment of the broad spectrum of critical functions executed by NF-kappaB in lymphocytes could not be obtained by the analyses of mice harboring disruptions of individual NF-kappaB genes, because of the concomitant deficiency of NF-kappaB in cells of the innate immune system and the redundant functions of NF-kappaB complexes. We have developed a model system where, due to the simultaneous loss of highly homologous subunits p50 and p52, the repertoire of NF-kappaB dimers is profoundly perturbed. We propose to use this model in a direct and systematic evaluation of the biologic impact of NF- kappaB factors in the development and function of B and T lymphocytes. The proposal has three Specific Aims. In Aim 1, we will address the role of NF-kappaB in B lymphopoiesis. NF-kappaB complexes are required for the establishment of the long-lived peripheral B cell pool By combining an in vivo and an in vitro approach, we will determine the mechanism(s) through which NF- kappaB controls this maturation step, and will begin by evaluating the physiologic relevance of previously proposed roles of NF-kappaB in B cell survival, activation, and mitogenesis. Defining the contribution of the transcript factor to B cell selection may also offer insights into autoimmune mechanisms and treatments. In Aim 2, we will examine roles of NF-kappaB in homeostasis of T cells. The physiologic relevance of previously suggested roles of NF-kappaB in costimulation and activation- induced cell death (AICD) will be determined by examining responses of NF-kappaB deficient T cells to their natural antigen, when instructed by an intact innate immune system. To further assess potential broad roles of NF-kappaB in autoimmunity and chronic inflammation, self-antigen-driven responses of these cells will also be examined. In Aim 3, we seek to identify and characterize a subset of NF-kappaB regulated genes that act to antagonize apoptosis. To achieve this goal we have successfully optimized an experimental protocol for the functional screening of mammalian libraries. This is of interest since these genes appear to be integral effectors of survival programs activated by costimulatory pathways in B and T cells, and may be directly involved in the generation of mature B cells and T cell homeostasis. In addition, NF-kappaB regulated pro-survival genes have been implicated in tumorigenesis, and resistance of tumor cells to chemotherapy and ionizing radiation, thereby providing potential new targets for cancer therapy.